A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A key performance parameter of any lithographic process, particularly one used for the manufacture of semiconductor devices, is the so-called overlay. Overlay is the accuracy (or the error) to which features in an applied pattern can be positioned directly on top of cooperating features applied to the same substrate in an earlier step. Modern lithographic processes may apply many measurements, modeling and correction steps to eliminate sources of error in the positioning of features, to achieve overlay of only a few nanometers. As the performance of lithographic apparatuses improves, reticle deformations, caused by for example clamping stresses, sagging, and reticle heating during exposures, are becoming a limiting factor for overlay improvements.
Reticle deformations due to clamping are kept as small as possible, by the clamping design. A U.S. Pat. No. 6,277,532 describes methods for mapping distortions of a pattern across a reticle, and subsequently correcting these for distortions induced by clamping. In another development, focus deviations due to non-flatness of the reticle (deformation in the Z direction) are compensated with Reticle Shape Correction (RSC). RSC is described for example by Z. G. Chen, K. Lai, K. Racette in “Optical error sensitivities of immersion lithography” Proc. SPIE volume 6250, SPIE CID number 652013. RSC uses additional marks to measure Z position at several points along each side of the image field. Low-order nonlinear height deviation of the reticle surface can be corrected. However, when reticle heating is taken into account, the effect of thermal stresses on the distortion of the pattern in the plane of the reticle (X- and Y-directions) becomes significant, causing non-uniform and non-linear movements of different portions within the pattern. Known processes do not provide for the measurement, let alone the correction of such in-plane distortion. To include additional marks over the reticle would impinge upon the product pattern itself and create problems for product designers. The time required to measure additional marks across the pattern would also tend to reduce throughput of the lithographic apparatus.
Accordingly, although some modern lithographic apparatuses have correction mechanisms (in software) that could be applied to compensate for higher-order distortions in the plane of the reticle, the means to measure those distortions is not readily available. It is considered to provide sensors to remotely sense the actual temperature across the reticle, and to estimate the consequent distortions so that they may be corrected. This generally requires additional sensors and space in the apparatus housing, however.